Comparative analysis of shallow and deep groundwater chemistry, focusing on spatial distribution and assessment of associated health risks at the south-central coastal areas of Bangladesh Open Access

Water is essential for life, influencing individual well-being, economic development, and ecosystem preservation (An et al. 2014; Goswami & Bisht 2017). Groundwater, a substantial freshwater reservoir, is widely used for domestic, irrigation, and industrial purposes, mainly in arid, semi-arid, and sub-humid areas (Kumari et al. 2014). Moreover, it is considered a desirable option for potable water because of its availability and relatively high resistance to contamination (Bhuiyan et al. 2016). Nevertheless, with rapid population growth and economic expansion, groundwater contamination has emerged as a pressing concern that threatens public health and exerts significant pressure on global groundwater reserves (Su et al. 2019; Shukla & Saxena 2021). In Bangladesh, approximately 95% of the rural and 70% of the urban population rely on groundwater as their primary source for drinking and domestic needs (Chakraborty et al. 2022).

Like other developing countries, Bangladesh faces a significant water pollution concern stemming from rapid urbanization and industrialization (Zahid & Ahmed 2006; Sarker et al. 2021). The release of untreated domestic and industrial waste into water bodies, coupled with the extensive use of pesticides and fertilizers in agriculture, increases the level of toxic metals in surface water (Bhuyan et al. 2017; Hasan et al. 2019; Shammi et al. 2023). As groundwater quality depends on recharge water quality from surface water sources (Kumar et al. 2014), vertical leakage and infiltration of contaminated water results in groundwater contamination, posing threats to public health (Foster 2001; Abdalla & Khalil 2018). Additionally, different hydrogeochemical processes, including oxidation/reduction, cation exchange, precipitation of secondary minerals, and saltwater intrusion, also govern the chemical characteristics of groundwater (Bhuiyan et al. 2016; Islam et al. 2018; Rafiq et al. 2022).

Coastal areas in Bangladesh are densely populated, and this rapid expansion is associated with the presence of fertile floodplains and the availability of inland waterways (Streatfield & Karar 2008). The coastal region covers almost 29,000 km² of land area and more than 30% of the cultivable lands of the country (Coastal Zone Policy 2005; Islam et al. 2017). Seawater intrusion in coastal aquifers due to the depletion of groundwater level and climate change with arsenic (As) contamination at shallow depths limits the availability of safe and freshwater options for the communities in the coastal regions (Zahid et al. 2018; Hasan et al. 2021; Hossain et al. 2024). Statistics reveal that almost 53% of these areas are affected by salinity due to seawater intrusion and entrapment of paleo-salinity (Hasan et al. 2018). Saline water intrusion intensifies during the dry season due to declining hydraulic head, with saline water extending as far north as Magura, approximately 240 km from the coast (Rasul & Chowdhury 2010). The situation is exacerbated due to excessive water withdrawal to meet ever-rising demand.

Water quality is a paramount concern, given its profound impact on agriculture and human health. Using low-quality water for irrigation leads to soil infertility, which introduces crop toxicity (Lakhdar et al. 2009; Bortolini et al. 2018). Furthermore, it detrimentally affects the operational efficiency and lifespan of irrigation equipment, including pumps and tubewells (Ayers & Westcot 1985). However, exposure to contaminants in drinking water carries a range of health risks, including the potential for chronic diseases like cancer and cardiovascular problems, adverse effects during pregnancy, and consequences for children's health, such as disruptions in neurodevelopment (Sharma & Kumar 2019; Xu et al. 2022; Shetty et al. 2023). Similar to other parts of the country, in the Barguna district, groundwater is a massive source of drinking water. The study area faces significant vulnerability to saline water intrusion, affecting the availability of fresh groundwater for drinking and irrigation purposes. Statistics reveal that 80% of the district's population faces salinity issues with their drinking water, 60% encounter difficulties in accessing fresh groundwater for domestic use, 40% suffer from various health problems due to poor water quality, and a staggering 90% of cultivated areas grapple with salinity-related challenges (Ashraf et al. 2020). Ensuring a secure and sustainable water supply for the residents of Barguna district necessitates imperative water quality assessments. Simultaneously, conducting health risk evaluations is crucial to minimize health risks.

Worldwide, several studies regarding groundwater quality and potential health risks due to urbanization (Xiao et al. 2020), industrial and agricultural activities (Li et al. 2016), and mining activities (Su et al. 2018; Wu et al. 2019; Samal et al. 2022) have been conducted. Several studies have explored various aspects of groundwater quality, hydrochemical properties, and health risks across different regions of Bangladesh. For instance, Rahman et al. (2018) delved into nitrate contamination, while Rahman et al. (2021) investigated the spatiotemporal distribution of boron within the coastal aquifers of Bangladesh. In Barguna district, Akter et al. (2019) assessed irrigation water quality, while Roy et al. (2017) identified groundwater quality and associated hazards in a more localized zone within Betagi Upazila of Barguna district. However, there is still a need for a detailed and comparative study investigating the groundwater quality in both shallow and deep aquifers, particularly in the Barguna district. Furthermore, to the best of our knowledge, there is no existing study that delves into the potential health risks associated with drinking in the Barguna district, Bangladesh. This study addresses this gap by providing an in-depth analysis of water quality and associated health risks across aquifer depths. The specific objectives are to compare shallow and deep tubewell (DTW) water based on (1) major ion concentrations, (2) spatial variations in ion concentrations, (3) hydrochemical facies, and (4) drinking water suitability, following guidelines established by the World Health Organization (WHO) and the Bangladesh water quality standards (BDS). Additionally, the study will recommend an optimal aquifer depth for safe drinking water.

From January–February 2022, 44 groundwater samples were collected from existing groundwater wells at the locations indicated in Figure 1. Of these, 30 were from shallow tubewells (STW) less than 200 m deep, and 14 were from DTWs exceeding 200 m deep. During water sampling, physicochemical parameters, including pH, EC, and temperature (°C), were measured on-site using the HANNA pocket meter and OHAUS portable EC and pH meter. The instruments were always calibrated with standard calibration solutions, and measurements were recorded and stored after obtaining stable readings from the respective devices. Water samples for Cl and HCO₃ were collected in acid-washed, deionized, water-rinsed plastic bottles without filtration or acidification. Water samples for major cations and metals, including Fe, Mn, and As, were collected in the same plastic bottles after being filtered using a 0.45 μm filter and preserved in 15 mL plastic vials containing 1% concentrated HNO₃. Water samples for SO₄ and PO₄ were collected after filtration without acidification. In the laboratory, major cations Na, Ca, Mg, K, and geogenic elements Fe, As, Mn, were analyzed using inductively coupled plasma mass spectrometry. In contrast, Cl and HCO₃ were determined using the titrimetric method, and the rest of the anions, SO₄ and PO_4, were analyzed using the ion chromatography method.

Among the 44 analyzed groundwater samples, approximately 80% (35 samples) exhibited EB within 6%, indicating their suitability for interpretation. The remaining nine samples displayed EB within 6%–15%, which was also considered acceptable for this study.

In this study, we used ArcGIS (ESRI) software to map the spatial distribution of water quality parameters. Specifically, we applied the inverse distance weighted (IDW) interpolation method while mapping physicochemical and hydrochemical parameters using the Spatial Analyst Extension of ArcGIS 10.4. The IDW interpolation principle involves a weighted linear combination of sample points, using statistical and mathematical techniques to predict values for unmeasured points. A Piper diagram was produced using Microsoft Excel 2019 to visually represent the concentrations of various ions in the water samples and to determine the water types. To assess potential health risks, we compared the concentration levels of each parameter against the water quality standards set by both the WHO and Bangladesh.

The results of the 44 groundwater samples analyzed for physicochemical parameters and laboratory-analyzed elements are presented and discussed here.

The spatial variations in temperature, pH, and electrical conductivity (EC) between shallow and DTWs are presented in Figure S1 in the supplement. A comparatively higher pH range (7.3–8.1) observed in most groundwater samples of the deep aquifer can be attributed to the presence of carbonates and bicarbonates due to the dissolution of carbonate minerals, e.g., calcite, dolomite, under acidic conditions (Appelo & Postma 2004; Helal Uddin et al. 2011). The relatively lower pH values (6.9–7.6) of STWs indicate a circum-neutral condition of the aquifer (Nelson 2002). The concentration of dissolved ions is often reflected as EC. EC values ranged from 1,020 to 2,375 μs/cm for DTWs. The lowest EC value of 1,020 μs/cm was observed in the central part, whereas the highest value of 2,375 μs/cm was observed in the northern part of the study area. EC values range from 918 to 37,300 μs/cm in the STWs. The northeastern part exhibited a relatively lower EC range (1,023.96 μs/cm), whereas a higher range (37,153.8 μs/cm) was observed in the southwestern and southern regions. The study area shows a distinctive pattern of increasing EC values from north to south, which suggests that the STWs are more likely to be affected by the intrusion of saline water (Mahmuduzzaman et al. 2014). On the other hand, the lower EC values in the southern region suggest that the DTWs are less vulnerable to seawater intrusion.

In deep confined aquifers, the dissolution of carbonate minerals like calcite and dolomite and the weathering of silicate minerals release HCO₃, Ca, Na, K, and Mg. Subsequently, this HCO₃ can react with excess H+ and neutralize them, resulting in a higher pH range (Biswas et al. 2012; Saha et al. 2019). However, several processes can occur in shallow aquifers, including contaminant leaching, decaying of organic matter, mineral weathering, denitrification, and reverse ion exchange which control the chemistry of the groundwater (Biswas et al. 2012; Rajmohan & Prathapar 2016). Degradation of organic matter in shallow aquifers can release acids that may cause a decrease in pH (Impellitteri et al. 2001). Additionally, those cations in groundwater react with HCO₃ and eventually form carbonic acid, resulting in reduced pH value. This highlights the noteworthy variations in physicochemical parameters between shallow and deep groundwater in the study area.

Figure 2 shows a wider range of chloride (Cl) concentrations in the DTWs, ranging from 26.6 to 552.6 mg/L. The northern half of the area has an exceptionally higher Cl range, with the central region displaying a much lower level of 26.6 mg/L (Figure 2). The remaining parts of the study area maintain an average Cl concentration (Figure 2). Figure 2 shows that the Cl concentration ranges from 73 to 16,200 mg/L in STWs. In the southeastern part of the Barguna Upazila, Cl concentrations reached as high as 16,200 mg/L for STWs. In contrast, the northeastern part of this upazila displayed a significantly lower Cl value of 73 mg/L. In shallow and deep groundwater, this study observes a consistent distribution pattern between Cl and EC, with higher concentrations of both EC (1,030–37,140 μs/cm) and Cl (73–16,195 mg/L) found in STWs (Figure S1 in the supplement and Figure 2). These parameters reach their peak values in the southern part of the study area for STWs, while these values are higher in the northern region for the DTWs (Figure S1 in the supplement and Figure 2). This phenomenon may be due to paleo-salinity or seawater intrusion in shallow groundwater (Ayyandurai et al. 2022). Seawater intrusion is directly influenced by factors such as the geological history of water-bearing formations, the hydraulic gradient, the rate of groundwater extraction, and the rate of groundwater replenishment (Calvache & Pulido-Bosch 1997; Ketabchi et al. 2016). In this region, the high abstraction rate of shallow groundwater for irrigation and other purposes (Kabir et al. 2020), increased soil salinity due to coastal flooding (Chen & Mueller 2018; Afjal Hossain et al. 2021), and infiltration of surface saline water have led to the elevated Cl level in shallow aquifers (Sarker et al. 2022).

In the DTWs, HCO₃ concentrations varied between 461.3 and 989.4 mg/L. A higher HCO₃ concentration of 989.4 mg/L was observed in the northern part of the study area (Figure 2). The remaining parts of the study area show moderate to lower HCO₃ concentrations with an average value of 458.71 mg/L (Figure 2). In the coastal region of Bangladesh, deep aquifer HCO₃ concentrations might be influenced by various hydrogeochemical processes, including the weathering of rock-forming minerals such as calcite and dolomite, as well as ion exchange (Rahman et al. 2011). Compared with DTWs, HCO₃ concentrations at STWs ranged from 36.7 to 1,372.1 mg/L, where the highest concentration of 1,432.46 mg/L was observed in the northern part (Figure 2). Such substantial dissolved HCO₃ in groundwater is more likely to be attributed to the biodegradation of organic matter and dissolution of carbonate minerals in shallow aquifers (Anawar et al. 2003; Zheng et al. 2004; Halim et al. 2009; Mahanta et al. 2011). The rest of the study area exhibited average HCO₃ concentrations of 300–500 mg/L. Most regions in Figure 2 displayed a relatively higher SO₄ range (0.0–14.4 mg/L) in the STWs, and the northernmost region area displayed a higher SO₄ concentration of 14.4 mg/L. In contrast, DTWs showed comparatively lower SO₄ concentrations, between 1.04 and 5.3 mg/L. The occurrence of SO₄ in STWs mainly resulted from the oxidation of SO₄-bearing minerals like pyrite or gypsum dissolution (Moncaster et al. 2000; García et al. 2001). Higher SO₄ concentrations of ∼ 14.37 mg/L in STWs may have resulted from SO₄-bearing fertilizers (ammonium sulfate, potassium sulfate, potassium magnesium sulfate) and pesticides used for agricultural purposes (Redwan & Abdel Moneim 2016).

Among the cations, Na was the predominant one, with concentrations ranging from 95.1 to 2,908.8 mg/L in the STWs and 127.04 to 513.3 mg/L in the DTWs (Figure 3). The sources of Na in groundwater are more likely to be attributed to the dissolution of minerals like halite and silicates (e.g., albite) and cation exchange processes (Jalali 2005; Saha et al. 2020). Additionally, elevated Na concentrations in the southernmost shallow groundwater (see Figure 3) indicate the potential inward movement of saline water from the sea (Sarker et al. 2021). The concentrations of Ca ranged from 10.03 to 198.2 mg/L in the STWs and from 9.6 to 38.4 mg/L in the DTWs, while Mg levels varied from 9.8 to 672.2 mg/L in the STWs and from 4.2 to 20.2 mg/L in the DTWs (Figure 3). Both Ca and Mg levels are higher in STWs compared with DTWs (see Figure 3). In shallow groundwater, the Ca level tends to be high in the northern region, whereas the Mg level is high in the southern part of the study area (see Figure 3). These cations, Ca and Mg, likely originate from the dissolution of carbonate, silicate, and sulfate minerals (Rahman et al. 2011). Figure 3 shows that the K concentrations ranged from 3.24 to 175.42 mg/L in the STWs and from 2.92 to 19.77 mg/L in the DTWs (Figure 3). Potassium (K) in groundwater can be attributed to various processes, including weathering K-feldspar, cation exchange, anthropogenic contamination from fertilizers, wastewater, and organic waste (Griffioen 2001). The relatively low K concentrations observed in DTWs are due to the weathering resistance of many K-bearing minerals (Saha et al. 2020). Conversely, the higher K concentrations found in the southern part of the STWs (see Figure 3) could be influenced by the composition of seawater and the use of fertilizers for agricultural purposes (Singaraja et al. 2015).

Among the hydrogeochemical processes that govern the concentration of As, Mn, and Fe in groundwater is the reductive dissolution of Fe and Mn-bearing minerals in the presence of organic carbon found in the Holocene sediments of the Bengal basin (van Geen et al. 2004; Saha et al. 2019). Microbially mediated redox reactions, influenced by organic matter, can mobilize Fe, Mn, and As into groundwater (Mladenov et al. 2015; Annaduzzaman et al. 2021). Furthermore, the effect of salinity, as indicated by an increase in EC, results in heightened ionic strength and a diminished activity coefficient, thus facilitating the dissolution of additional Fe and Mn into groundwater (Rakib et al. 2020). Consequently, this phenomenon may release adsorbed As through the oxidation and reduction of Fe-bearing minerals such as Fe-(oxyhydr)oxides, which significantly modify the composition of groundwater (Bose & Sharma 2002; Battistel et al. 2021).

The assessed physicochemical parameters, major ions, and geogenic elements were compared with WHO and Bangladesh water quality standards (BDS) to identify potential health risks for the shallow and deep groundwater of the studied areas. The samples that crossed the water quality standard are presented in Table 1. All water samples collected from STWs and DTWs surpassed the BDS and WHO standards for EC, indicating severe salinity contamination. (Rakib et al. 2020) highlights significant drinking water scarcity in coastal communities, which is mainly due to salinity, with around 78%–96% of the population depending on local ponds and rainwater. The survey on local perceptions conducted by this author reveals that worries regarding arsenic (As) and iron (Fe) contamination in groundwater are relatively less pronounced than concerns associated with salinity.

All the samples (30 STWs and 14 DTWs) exceeded the BDS and WHO limits for EC. In STW Cl concentration, 17 surpassed both BDS and WHO limits, with an additional 13 samples exceeding the WHO limit. Eight samples of the DTWs crossed the WHO value. Although elevated amounts of chloride can negatively affect plant development (Wang et al. 2020) and the structural integrity of metallic pipelines and buildings (Tang & Edwards 2017), there is currently no empirical data indicating any detrimental health effects on humans resulting from chloride ingestion. For bicarbonate, eight STW water samples exceeded the health risk threshold, with half surpassing BDS and WHO values and the remaining half exceeding only the WHO value. In the case of DTW, only two samples were within the safe limit. There is a lack of conclusive evidence regarding the health effects associated with the presence of bicarbonate (Brindha & Kavitha 2015). A significant number of samples showed elevated levels of Na (27 STWs and 11 DTWs) and K (17 STWs and six DTWs) that exceeded the limits set by both WHO and BDS. However, for Ca (12 STWs) and for Mg (26 STWs) while many samples exceeded the BDS limits, the majority of them remained within the acceptable limits defined by the WHO. Elevated concentrations of calcium and magnesium leading to water hardness may pose health risks, including the potential for kidney stone formation, arterial calcification, and various cardiovascular and stomach disorders (Brindha & Kavitha 2015). Among the geogenic components, iron (Fe) levels in the research area's water samples (25 STWs and two DTWs) exceeded both BDS and WHO standards. It is important to note that the WHO's suggested iron levels are primarily related to aesthetic concerns, such as colour, flavour, and odour, rather than health criteria. High iron concentrations are commonly found in reducing groundwater conditions. Regarding Mn, seven samples of STW crossed the WHO recommended value, while 22 samples crossed the BDS recommended value. High intake of Mn through drinking water can cause diseases like bronchitis, impotence, and Parkinsonism (Zoni et al. 2007). Newborns are particularly vulnerable, as elevated Mn levels can disrupt intellectual development during pregnancy. Arsenic levels in the water samples ranged from bdl to 55 μg/L. Eighteen samples from STWs exceeded both the BDS and WHO limits, while all DTW samples remained As-safe. Contamination by arsenic affects nearly 79% of aquifers in the southwest coastal region, significantly impacting both agricultural sustainability and food security (Abedin et al. 2012). Ingesting arsenic-contaminated drinking water has gradual health effects, as chronic arsenic poisoning, known as arsenicosis, heightens the risk of various health hazards. These include skin lesions, cancers, restrictive pulmonary disease, peripheral vascular disease, hypertension, and heart disease. Skin changes resulting from arsenic poisoning manifest as a distinct raindrop pigmentation pattern (Edmunds et al. 2015).

The findings of this research will aid policymakers in making informed decisions regarding freshwater management in the coastal regions of Bangladesh. Through a comprehensive analysis of water quality for STWs and DTWs, the study demonstrates that groundwater collected from shallow tubewells is relatively unsuitable for human consumption. In contrast, groundwater from DTWs is comparatively safe for drinking. The cost disparity between shallow and deep tubewells necessitates that communities comprehend the severity of contamination threats associated with consuming water from these sources. This conclusion serves as a significant awareness message for local communities that depend on shallow tubewells for their drinking water and for social welfare organizations that can assist in educating residents about the potential health risks associated with shallow well water. A significant contribution of our research is the comprehensive analysis of ion concentration ranges, establishing a valuable baseline for subsequent studies. This information is essential for developing targeted remediation strategies and ensuring safe water utilization. Furthermore, the data obtained from shallow tubewells can facilitate additional evaluations of water suitability for alternative purposes, such as irrigation and industrial applications, thereby guiding decision-makers in selecting appropriate good depths for specific uses. Our research emphasizes the health risks associated with distinct water quality parameters, offering a thorough assessment of their spatial correlations and variations with depth. In the long run, this comprehensive approach guarantees that water quality management efforts are not only informed by data but also customized to mitigate localized contamination risks effectively. By incorporating spatial and depth-based analyses, we have established a vital framework that policymakers, researchers, and water managers can employ to protect public health and enhance sustainable water resource management.

This study focuses on the south-central coastal region of Bangladesh, where residents face a severe freshwater crisis caused by saline water intrusion and/or paleo-salinity and arsenic contamination. The outcomes of this study offer essential insights into the chemical composition of water in STWs and DTWs, revealing potential health risks associated with water consumption. The spatial distribution of major ions shows distinct patterns in STWs and DTWs. In STWs, the northern region has higher Ca, Fe, and HCO₃ concentrations, while the southern region shows elevated Cl, Na, and K levels. On the other hand, higher concentrations of HCO₃ were observed in the DTWs compared with STWs. In DTWs, the northern region has higher concentrations of Cl, HCO₃, Na, and Mg, while the central region shows notably higher levels of SO₄, K, and Ca. Cl dominates STWs with NaCl-type waters, while DTWs are rich in HCO₃ and mostly exhibit Ca-HCO₃-Cl and Na-HCO₃-type waters. The higher EC and concentrations of Cl and Na in the shallow groundwater indicate the intrusion of saline water and/or paleo-salinity. Conversely, the deep groundwater, with its prevalence of HCO₃ and lower levels of Na and Cl, appears less influenced by seawater intrusion and is free from arsenic contamination. Comparing the results of the current study with the water quality standards set by the WHO and BDS, it can be concluded that deep groundwater is comparatively safe for consumption. This study adds to the knowledge about groundwater quality in the Barguna district and is a valuable resource for the government, local communities, and policymakers. It offers essential insights to inform decision-making processes and strategies to secure a sustainable and safe water supply for this vulnerable region, addressing both immediate challenges and long-term sustainability.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.